Sonic is a guy with supersonic speed, and this might prove to be a little annoying to some people. Should he choose to use his speed to help, annoy, interfere with other people, that decision is up to you!

Also, due to the fact he's so fast, he's able to dodge quite a few things. Again, let me know how this sits with you.

On the flip side, here are some things I think I should add in for Sonic's case.

can I kiss him? You can try, but he'll evade it like the wind.
can I kill him? No, unless we've discussed it prior.
what languages does Sonic speak? Since I'm playing the dub one, and considering Station Square is somewhere in the states, I'm sticking with English only. He says 'sayonara' once though.
can I trip him? ... yes, if you have really good timing. But remember, he travels at a very intense speed- Make sure tripping him doesn't break your leg!
what do you think about fourth-walling? Love it. Considering who Sonic is, you can totally have your character flip out about Sonic if they would recognize him.